Currently available EPROMs have access times in the 150-200 nanosecond range. As processing circuits have increased in speed, it is desirable that the access rate for EPROMs also be increased.
The access time for EPROMs is particularly important in the field of Electrically Programmable Array Logic, otherwise known as EPAL. In EPAL, the speed at which the logic can function is dependent on how fast the internal EPROM can be read. Thus, the reading speed of the EPROM has a direct correlation to the speed of the EPAL.
Previous solutions to increase the access rate of EPROMs have produced deleterious side effects. The main impediment to increasing EPROM speed is control of "hot electrons," which pass through the oxide above the depletion region into a floating gate. To program the EPROM, hot electrons are purposely forced into the floating gate to create a charge therein. However, during read operations, it is undesirable for hot electrons to penetrate into the oxide region or the floating gate, since the accumulated effect of the passing of hot electrons during read cycles will result in a charge being stored in floating gates which were not programmed, causing "read disturbs."
Although using a high driving current will increase the speed with which an EPROM can be read, the higher driving current produces more hot electrons, resulting in an increased rate of read disturbs. Furthermore, hot electrons can be caught in the oxide between the depletion region and the floating gate, causing breakdown of the oxide region over a period of time.
Previously developed methods of preventing false programming when using higher driving currents have resulted in EPROM cells with increased complexity, and increased size. The increased size of the EPROM results in less space remaining on the semiconductor chip in which to form the array logic. Therefore, currently developed EPALs face a trade-off between logic density and operating speed.
Thus, a need has arisen in the industry for an EPROM structure which allows fast reading access time without substantially increasing the size of the EPROM memory cells.